Bug 1348380 - Add param and user setting to control elasticsearch behavior

Bug 1348380 - Add param and user setting to control elasticsearch behavior

1) add an 'elasticsearch' param (configured via admin.cgi) that turns off all
code that would attempt to connect to elasticsearch.

2) add a user preference that controls using elasticsearch for searches, which defaults
to off for logged-in users. Anonymous users will default to using elasticsearch
for the greater good. Note that elasticsearch, if available, will still power
user-autocompletion (unless turned off as by the above parameter).